1. Field of the Invention
The present invention relates to a top spin valve sensor that has a free layer structure with a cobalt iron boron (CoFeB) layer for promoting a softer free layer structure with improved easy axis and hard axis coercivities.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer rotates from a position parallel with respect to the magnetic moment of the pinned layer to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
A read head in a magnetic disk drive of a computer includes the spin valve sensor as well as nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is first formed followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located at the bottom o the sensor next to the first read gap layer or at the top of the sensor closer to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic leers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel.
A free layer structure typically employs a nickel iron (NiFe) free layer and a cobalt iron (CoFe) layer which is sometimes referred to as a nanolayer. Typical thicknesses are 45 xc3x85 for the free layer and 10 xc3x85 to 15 xc3x85 for the nanolayer. While the nickel iron (NiFe) free layer has soft magnetic properties, the cobalt iron (CoFe) of the nanolayer unfortunately increases the uniaxial anisotropy HK and the easy axis coercivity HC of the free layer structure making the magnetic moment of the free layer structure stiffer and less responsive to signal fields from the rotating magnetic disk. The result is less sensitivity of the read head. The uniaxial anisotropy HK is the amount of applied field required to rotate the magnetic moment of the free layer structure in a direction 90xc2x0 from its easy is and the easy axis coercivity HC is the amount of applied field required to rotate the magnetic moment of the free layer structure from one direction along its easy axis to an opposite direction.
The cobalt iron (CoFe) nanolayer also causes the free layer structure to have a higher hard axis coercivity HCK where the hard axis coercivity is the amount of field required to return the magnetic moment of the free layer structure from a remanent magnetization to zero. Hard axis coercivity HCK can be seen on a hard axis B/H loop of the free layer structure by the amount of openness of the hard axis loop along the abscissa wherein the abscissa represents the applied field. When the free layer structure has hard axis coercivity noise is generated in the read head due to jumps in the movements of the magnetic moment of the free layer structure in contrast to a smooth transition when subjected to signs fields from the rotating disk. Accordingly, it is desirable to improve the softness of the free layer structure so as to reduce uniaxial anisotropy HK, easy axis coercivity HC and hard axis coercivity HCK.
I have provided the free layer structure with a layer of cobalt iron boron (CoFeB) which improves the performance of the free layer structure. Tests conducted by me show that the uniaxial anisotropy HK, the easy axis coercivity HC and the hard axis coercivity HCK of the free layer structure were improved. The preferred pinning layer is platinum manganese (PtMn). The invention can be employed for a top spin valve sensor with a single or AP pinned layer structure.
An object of the present invention is to provide a seed layer for a top spin valve sensor that improves the soft magnetic properties of a free layer structure.
Another object is to provide a seed layer for a top spin valve sensor which improves the uniaxial anisotropy HK, easy axis coercivity HC and hard axis coercivity HCK of a free layer structure.
A further object is to provide a seed layer for a top spin valve sensor which improves the texture of layers constructed on the seed layer for improving performance of the spin valve sensor.